Marine worms come in a staggering variety of forms. New
forms are being discovered all the time and here we can only
look at a few selected examples. We begin with the
polychaetes (btistle worms). Polychaetes, like the earthworms
are segmented (annelid) worms. Earthworms, leeches and
some freshwater worms belong to a group called the
oligochaetes, since they have only a few spines (chaetae) per
segment. Polychaetes have more chaetae per segment and
are mostly marine (though some freshwater and terrestrial
forms occur). polychaetes come in a vast variety of types and
many are large and spectacularly coloured.
The pictures above are of a computer model of one type of polychaetes called Terebellids. Terebellids
have many feeding tentacles attached to the head end, earning them the common name of spaghetti
worms. The smaller red tentacles are called branchiae and act as gills. These gills typically occur in two
or three pairs and are usually branching tentacles. The worm absorbs oxygen from the surrounding
water through these red gills. These worms are sedentary burrow-dwellers, occupying shallow or vertical
burrows in sand or mud and lined with mucus secretions, or constructing membranous tubes covered
with sand, mud, etc. and buried or fixed to stones and plants, etc. many of these worms are large and
thicker than earthworms of the same length, e.g. Amphitrite johnstoni is up to 25 cm long and has
90-100 segments. The tentacles stretch out from the burrow to obtain food, they will flatten and creep
over the sand (by means of cilia on their under-surface) and secrete sticky mucus to which sticks
organic detritus sifted from the sand. These food particles are passed along the tentacle, along a gutter
that forms along each tentacle, to the mouth and ingested, or the tentacles may be wiped across the lips
of the mouth to remove the food particles. These worms are selective deposit feeders - rather than
swallowing the sand/mud and ingesting what food particles happen to be in it, they sift the sand first with
their tentacles and select suitable particles for ingestion. The tentacles of some Terebellids may extend
for a metre or more in length.
Left: a computer model of a polychaete based
on Eunice aphroditois, the bobbit worm. These
worms eat algae, crustaceans and other
worms. These worms frequently reach 2 to 3
metres in length, but 6 metre giants have been
reported. These larger specimens may be 100
years old. A one metre specimen is about 2.5
cm (one inch) in width. It searches for food with
its front end at night, keeping the rear
anchored in its burrow. It is reported that they
will frequently try to bite divers, and looking at
those fierce jaws I am sure they can give quite
a nasty bite! Indeed, they strike prey with such
speed and force that they can slice a six inch
fish in half! Apparently, the common name of
Bobbit Worm is taken from a woman of the
same name who cut off the penis of her
husband, because it is said that the Bobbit
Worm cuts off the male's penis some time after
mating and feeds it to her young. However, like
most polychaetes, the Bobbit worm relies on
external fertilisation and so naturally lacks
external genitalia. The parapodia of these
worms form the leg-like projections appended
to each segment. These parapodia are not true
legs since they contain no joints (other than the
one at the base where the appendage meets
the body). Neverthelss, they may be used for
crawling and in some polychaetes they are
modified into paddles for swimming (as in the
paddle worms Phyllodoce) or into tentacles.
The terebellid and Bobbit worm models are 3D
computer models rendered in Pov-Ray.
In Eulalia the parapodial paddles are leaf-like and these worms are a beautiful verdant green, hence
their common name of Green Leaf Worms. These more active types of worm, that crawl or swim freely
and only form temporary burrows if they burrow at all, have well-developed eyes and are often
spectacularly coloured and the head is equipped with sensory tentacles. In Nereis virens, the King Rag
Worm, which may be up to 20 cm long, the parapodia are used in both crawling and swimming and these
worms may be spectacularly coloured with the main body an iridescent purple or deep blue and the
parapodia have red bases and are tipped with yellow. When the tide goes out these worms can be found
burrowed into the sand, but they will emerge at certain times when covered with water in order to feed or
Palola, the Palola Worm, is a relative of the Bobbit Worm which can reach 3 metres in length and 30 mm
in diameter. These worms reproduce by forming special sexual individuals called epitokes. In Nereis, an
entire individual transforms into an epitoke, but in the Palola Worm the rearmost segments develop into
the epitoke and then detach to swim to the surface (these Palola epitokes are often over one metre in
length). These epitokes are packed with gametes and swarm to the surface all at the same time, typically
once a year on the night of a full Moon or at some other precisely timed part of the Lunar cycle. The
epitokes of some species luminesce and swim around each other creating circles of light. The epitokes
are packed with gametes and once they reach the surface they essentially 'explode' or erupt (with
sunrise in Palola) shedding their gametes. The sperm will then fertilise the eggs in the sea. The
synchronous swarming of the epitokes ensures that there is a high density of sperm to fertilise the many
eggs, increasing the chances of success. The fertilised eggs grow into strange tiny swimming larvae
called trochophores. Trochophore larvae are shaped like spinning tops and spin through the water,
swimming by means of a girdle of cilia. Some trochohores will feed on tiny plankton in the surface waters,
others are laden with yolk (as in Nereis, Palola and Eunice) and do not feed and swim close to the sea
bed. Eventually the trochophore sinks to the seabed and metamorphosis into a mature worm. Some
polychaetes take more care of their eggs, keeping them in their tube or burrow or brooding them within
Below is a diagram (click diagram to enlarge) of Neoamphitrite, a filter-feeding terebellid polychaete
(redrawn with a colour code from Dales' 1955 classic diagram):
Click the thumbnail above for an
unlabelled version of this diagram.
Neoamphitrite figulis (Amphitrite jonstoni) is about 1 cm in diameter and
up to 25 cm in length (for 100 segments) and is a marine terebellid.
Terebellids (also called spaghetti worms) are burrowers or tube-dwellers.
Neoamphitrite builds a thin tube of mud-encrusted mucus for itself.
Note the following structures:
- The numerous long tentacles (only the bases are shown)
- The 3 pairs of branchiae (gills)
- The anterior nephridia (the worm's equivalent of kidneys, there are
also posterior nephridia which are not shown)
- The dorsal vessel (the worm's equivalent of the heart) which pumps
blood into the various blood vessels, like the ventral vessel
- The gut, consisting of the mouth, bordered by a pair of lips, which
opens into the oesophagus, which leads into the stomach, which
leads to the intestine and anus
- The mucous glands of the ventral gutter of the head - used for tube
to blood vessels. Blood flows from the anterior to the posterior in the ventral vessel (situated immediately
beneath the gut) and back from posterior to anterior in the dorsal vessel. These longitudinal vessel contract
rhythmically, helping force the blood along. The branchiae contain blood vessels and may also contract
rhythmically to help circulate the blood. Lateral vessels (side-branches) carry blood from the ventral vessel to
the epidermis ('skin') and muscles (and to the branchiae in those segments that have them).
Some small polychaetes have no blood vascular system at all. All polychaetes, however, have a
fluid-containing coelom, or body cavity, which occurs betwwen the body wall and the gut and other organs.
This coelom is lined by an epithelium (an epithelium is a thin covering tissue that covers an external or internal
surface on an organ or body part) possessing cilia - microscopic hairs that beat rhthmically, circulating the
coelomic fluid. The smallest polychaetes lack respiratory pigments altogether (a respiratory pigment carries
oxygen around the body, in mammals like humans it is the haemoglobin carried in red blood cells which gives
the blood its red colour). Some, although lacking a blood system, have cells in the coelom (called
coelomocytes) that carry haemoglobin. Most have a closed blood system in which respiratory pigments, either
a red haemoglobin or a green chlorocruorin, is dissolved in the blood (but not carried by blood cells). Thus,
some polychaetes have colourless blood, some red blood and others green blood! Polychaetes have few
blood cells (they have a few circulating stray cells, possibly coelomocytes that have migrated into the
blood).Phagocytes - cells, such as white blood cells in mammals, that eat foreign material like bacteria, and cell
debris and so keep the animal clean, though most of their phagocytes occur principally in the coelom and so
are called coelomocytes.
P. Dales (a pioneer of studies on polychaetes) in 1964 wrote of Neoamphitrite figulis: 'The coelom is extensive,
and if a worm is punctured a considerable volume of fluid spurts out. This has an orange, Indian red, or brown
colour due to the coelomocytes it contains, and these may be so numerous as to give a creamy consistency to
the fluid,' (external link). These coelomocytes contain haemoglobin, in addition to about twice as much
haemoglobin existing in the blood. In older coelomocytes this red haemoglobin may break down to brown
haematin, giving some worms a brown colour. These coelomocytes also phagocytose foreign materials, but
Dale argued that they have a third and most major function - to carry fuel (glycogen and lipids) to the
developing gametes, and these coelomocytes increase in number in the summer when the worms are
manufacturing gametes ready for spawning. In the polychaete Nephtys coeca (external link) many of the
coelomocytes were derived from degenerating body wall muscle cells during autophagy (lit. 'eating oneself')
during starvation. The heart-body is an extensive lump of tissue in terebellids and is situated inside the dorsal
vessel near the branchiae (you can see it almost completely blocking the lumen of the dorsal vessel at its front
cut-end in the diagram) and is responsible for synthesising blood respiratory pigments and for disposal of
materials. The heart-body typically enlarges with age and has been seen to accumulate foreign material
originally ingested by coelomocytes (Braumbeck and Dales 1984). The heart-body may also have some
function in regulating blood flow as it blocks the dorsal vessel lumen in the branchial area when the vessel
The branchiae (gills) contain looped blood vessels and increase the surface area of those segments that bear
them, by up to one third. Polychaetes generally absorb oxygen from the water across their whole body surface,
but any appendage, such as branchiae, will increase this absorption. In terebellids the branchiae are dorsal
(on the back) and restricted to a few anterior segments (since the remaining segments are encased in the
tube, they are not able to absorb as much oxygen). In some polychaetes they are many of the segments along
the body length. In some polychaetes the branchiae may be branched and even pectinate (feathery, like the
gills of a fish). They are simply outgrowths of the body wall containing blood vessels for gas exchange and may
Terebellids are dioecious (they have separate sexes) and are not known to reproduce asexually. Males are
superficially indistinguishable from females, except near to spawning, when the eggs colour the gravid females
pinkish to green and the sperm make the males a cream colour. Immature gametes are released into the
coelom where they mature prior to being shed through the reproductive nephridia (nephridia are excretory
organs). Fertilisation is external and terebellids typically live for several years, spawning each year; some
polychaetes are annual - maturing in one or two years, spawning once and then dying.
Polychaetes tend to synchronise spawning to maximise the odds of fertilisation. Many synchronise according to
Lunar cycles and most individuals may spawn on a single night. Some polychaetes produce swimming
reproductive individuals, called epitokes. Epitokes may be produced by metamorphosis of an individual worm
or by asexual reproduction - developing from the rear segments (or budding off therefrom) of a worm before
detaching and leaving the tube or burrow. Epitokes are specialised for reproduction and are laden with
gametes. The female epitoke secretes a pheromone which attracts the male and triggers sperm shedding
which, in turn, stimulates shedding of the eggs by the female.
In many polychaetes the fertilised egg hatches into a free-swimming pelagic larva called a trochophore:
Note the nerves radiating from the apical organ, one of which connects to the eye before innervating the gut.
Protonephridia - excretory and osmoregulatory organs which drive out excess water and waste products
along with it.
The trochophore is pelagic (except in some species in which it remains within the egg) and in some species is
capable of feeding (as is the one in the 3D model above). The trochophore develops into a juvenile worm by
metamorphosis. Segments start to form between the topmost prototroch region (apical region, trochophore
and mouth) and the pygidium (telotroch and anal region). As the worm continues to develop, the front-most
pseudosegment, the prostomium, and the rearmost pygidium are directly derived by transformation and
growth of trochophore tissues, with adult segments inserted in between from a growth-zone just in front of the
pygidium, such that the rearmost segments are the youngest. The number of segments is more-or-less
precisely determined and depends on species.
Did you know?
Polychaetes have considerable powers of regeneration. In tube-dwelling peacock worms (sabellid worms) the
front-end is prone to predation and if as many as the first free head segments are lost then these will
regenerate as the cut end forms a growth zone called a blastema. If more segments are lost, then the front
most remaining ones will metamorphose into a new head! In this way a feeding head is restored as quickly as
possible. Similarly rear segments can be replaced. The key stimulus is the severing of the ventral nerve cord -
if the nerve cord is surgically cut then can develop into an extra head or tail according to where the cut is
situated! In Chaetopterus a single segment may regenerate into an entire worm!
You may be wondering why worms that sustain such injuries don't bleed to death or die of infection. Well, their
blood-system is at a much lower pressure than in mammals and so leaks can be plugged before blood loss is
total. Worms do have defensive systems to combat infection, though these are less well developed than in
mammals, however, they don't have the rich glucose and iron-carrying blood of mammals - such blood is
designed to nourish especially the large brains of mammals, but it is also excellent food for certain bacteria
(those that can resist the complement proteins and sequester the iron) especially if the wound is low in oxygen
due to damaged blood supply (this favours the growth of anaerobic bacteria that cause such infections as
gangrene). Invertebrates like polychaetes tend to have lower glucose concentrations, perhaps a tenth as high,
and transport sugars like maltose (some polychaetes) or trehalose (insects) instead - these sugars are harder
for bacteria to digest. (Plants transport sucrose for the same reason). Thus, invertebrates seem less prone to
Polychaetes may exhibit autotomy or 'self-amputation' in which a trapped body part is sacrificed, perhaps to a
predator in much the same way that a lizard sheds its tail. Fan worms may shed their whole crown of tentacles
and polychaetes may break in two. Once I was handling specimens of the king rag worm, Nereis virens. These
large polychaetes will crawl over your hands and generally do not bite unless agitated, but once I picked one
up by holding it at two points. Almost immediately the worm just fell apart into two living halves when I applied
so little tension that it was unlikely to break any worm unless it was designed to do so, thus it seems likely that
the worm sensed that it was trapped and so fragmented itself to attempt an escape.
The trochophore larva is about 0.1 mm in diameter.
Note the following structures:
Prototroch - an equatorial band of beating cilia that propels
the trochophore through the water.
Metatroch - in feeding trochophores this band of cilia beat in
a direction opposite to that of the prototroch and between
them they drive a current of water bearing food particles (e.g.
bacteria) into the food groove where cilia around the mouth
drives the particles into the gut.
Eyespot - typically 1-3 are present.
The name 'polychaete' means 'many chaetae'. Chaetae are bristles (sometimes called setae) usually
often in length, but often reaching considerable lengths relative to the worm body in some species.
Earthworms belong to the Oligochaeta ('oligochaete' meaning 'few chaetae) along with freshwater
oligochaetes. Other ploychaete appendages include tentacles, antennae, palps, jaws and teeth, fans
and paddles, branchiae, anal cirri, and parapodia (bearing tentacle-like cirri) and others. This vast
diversity of appendage is key to polychaete success, along with their segmental nature. A study of the
structure and function of these appendages is formidable and they are hard to visualise without
seeing actual specimens. Let's look at some of these appendages, starting with tentacles. The
spaghetti worm (terebellid) shown above certainly has an impressive crown of tentacles which are
used in feeding. (The mode of operation of these tentacles in feeding is very different from that of the
sabellid fan-worms or peacock worms which use their tentacles to sieve food from the water, so don't
confuse the two!).
A very different type of polychaete are those that use their parapodia to crawl (surface dwellers and some
active burrowers) and swim (pelagic polychaetes). A typical example would be the nereids, such as Nereis.
These are the so-called errant or wandering polychaetes. The computer model below shows the structure of a
typical nereid like Nereis. Nereis is an active burrower but also crawls and swims when it needs to.
These polychaetes are often large and often brightly coloured (sometimes with a metallic lustre) and may have
a nasty bite! The powerful jaws are situated inside the pharynx (throat) but become inverted when in use, when
the whole pharynx turns inside-out and protrudes through the mouth! The one above is doing just that. The
multiply lobed structures (shown in pink) are the parapodia. These are divided into two principle branches (they
manner similar to simple legs (except that they are not definitely jointed in the sense of arthropod legs, but are
soft muscular appendages) - by lifting the parapodia up and moving them forward, then by placing them on the
ground and moving them backwards the worm can advance by crawling. The parapodia have to move in an
organised manner for this to be efficient and they move in waves. When these worms need to crawl faster (and
they can crawl quite fast!) the body is thrown into S-shaped waves, rather like a snake crawling.
In swimming, the parapodia row through the water in waves as the body undulates like an eel, but with one
important difference - in all fish (?), including eels, the waves travel backwards, from anterior to posterior,
faster than the fish moves forward, pushing the fish forward; whilst in polychaetes the waves travel forwards,
from posterior to anterior, faster than the worm moves forwards. Intuitively one would expect such worms to
swim backwards! (They have a negative angle of attack). However, this apparent paradox was resolved when it
was realised that fish are smooth, whilst polychaetes have parapodia projecting from their sides which roughen
the surface, creating turbulence. Putting this into a mathematical model demonstrated that the waves have to
travel forwards in order for our rough worm to travel forwards! The rowing of the parapodia also provides
forward thrust. Nereis is not an especially good swimmer, and those forms that are better swimmers (and in
free-swimming reproductive forms) the parapodia are enlarged and greatly flattened into larger paddles.
Septum: intersegmental partitioning membrane
AM: Acicular Muscles (attached to aciculum)
DC: Dorsal Cirrus
DCM: Dorsal Cirrus Muscle
DGS: Dorsal Gut Suspensor
DLM: Dorsal Longitudinal Muscle
DP: Dorsal Parapodial muscles
DT: Diagonal Trans-septal muscle
DV: Dorsal Vessel
DVM: Dorso-ventral Muscle
IP1, IP2, IP3: Intrinsic Parapodial muscles
MD: Median Diagonal muscle
NC: Nerve Cord
VC: Ventral Cirrus
VLM: Ventral Longitudinal muscle
VP: Ventral Parapodial muscles
VV: Ventral Vessel
Click the picture above to enlarge it.
It is informative to compare the internal structure of an
errant polychaete like Nereis with an annelid oligochaete
earthworm like Lumbricus and relate this to the method of
locomotion used by each worm. In the earthworm body wall
there is a definite and complete cylinder of circular muscle
surrounding a complete cylinder of longitudinal muscle.
This is ideal for earthworm locomotion - for burrowing
through compacted terrestrial soils, which requires the
worm to build-up considerable hydrostatic pressures in
order to push its way through the soil; and also for their
mode of peristaltic crawling. In Nereis, the body-wall
musculature is necessarily broken-up by the presence of
the large locomotory parapodia, resulting in a more
When students of science are confronted with a situation as complex as the diagram above, there is a
tendency to skip the details, however, it is a useful exercise to analyse such a system and break it down into its
functional components. The key is to remember that muscles simply contract, so look at what's fixed to the two
ends of the muscle and visualise what would happen if that muscle shortened. For example, the dorsal cirrus
muscle (DCM) is clearly involved in moving the dorsal cirrus (a cirrus is a tentacle-like appendage, quite short
in this case, but long in some species). The intrinsic parapodial muscles (IP1, IP2, IP3) have both their
end-points situated within the parapodium, thus they are intrinsic to it. Contraction of IP1 will clearly flatten the
parapodium, contraction of IP2 will shorten the notopodium, whilst contraction of IP3 will shorten the
neuropodium - the intrinsic muscles change the shape of the parapodium.
The large dorso-ventral muscles (DVM, joining the back or dorsum to the front or ventrum) flatten the body of
the segment whilst the large dorsal longitudinal muscles (DLM) shorten the segment. Adjacent segments are
divided by muscular sheets or septa (intersegmental septa) which can seal-off the fluid within a single segment.
Since water is essentially uncompressible, flattening of the segment will tend to lengthen it, whilst shortening of
the segment will tend to heighten or widen it. This arrangement, in which contraction of one set of muscles
opposes or antagonises another is a common mechanism of controlling movement in animals. In this case,
antagonism is made possible by the coelomic fluid, which fills the segmental cavity and acts as a hydrostatic
skeleton. The size of this set of dorso-ventral and longitudinal muscles testifies to their importance - they
provide the main power in locomotion.
In earthworms the septa more tightly seal-off each segment, allowing each segment to shorten or lengthen
almost independently of adjacent segments (though there are valves in the septa which can release excess
pressure into adjacent segments). In many polychaetes, including Nereis, the isolation of hydrostatic pressure
within each segment is less important - these worms do not rely on shortening or lengthening of specific
segments for crawling, and the septa are generally incomplete (though they do form a tighter seal when
muscles contract during locomotion). However, hydrostatics still play an important part - when the longitudinal
muscles on one side of a segment contract, the segment bends into a curve, with pressure forcing the side
opposite the contraction (the side with the longer arc) into a turgid state (it's a bit like pinching one side of an
inflated balloon and watching the remainder stretch tight). When Nereis throws itself into S-shaped (sinusoidal)
curves during swimming or fast crawling, the longitudinal muscles on the inside of the curve contract,
shortening the segments on this side. The coelomic fluid is squeezed to the opposite side of the segments, on
the outside of the curve, distending them and their parapodia on the outside of the curve. This gives the
parapodia rigidity to work against the water (as paddles) during swimming or against the ground or the side of
the burrow, during their power stroke. If some pressure leeks out into adjacent segments, that's not a problem,
since adjacent segments must also change to some degree, creating smooth sinusoidal pressure waves.
The longitudinal muscles of errant polychaetes have been broken-up into muscle blocks, compared to the
complete cylinder of earthworms. This is because of the uneven force they have to exert - they need to
contract each side of the animal in alternate fashion. This tendency is developed to its maximum capacity in
fast-swimming fish. There is also a tendency in polychaetes for the intersegmental septa to be reduced and in
some parts of the worm they merely form suspensory ligaments which hold the gut in place - echoing the
mesenteries of vertebrates, like human beings.
Above: (B) during fast crawling and swimming, active polychaetes like Nereis throw their bodies into
sinusoidal S-shaped curves. The longitudinal muscles in segments inside the curve contract, shortening
the segments on one side and stretching them on the other, outer side of the curve. The parapodia on
the crests of the wave (outer curves on the distended side opposite the contraction zones) paddle
backwards (curved arrows), exerting thrust, during their power strokes. The main thrust during swimming,
however, comes from the force exerted by the undulating body on the water. In rapid crawling, the force
generated by the shortening of the longitudinal muscles is transmitted to the parapodia on the opposite
sides (outer curve) which contact the ground or burrow during their power strokes. The parapodia on the
contracted sides are raised as they move forward during their recovery strokes, in preparation for their
power strokes. The waves travel down the body from posterior to anterior (C) in the same direction as
locomotion (blue arrow).
The Evolution of Polychaetes
One of the reasons why invertebrate zoology should be still be taught (and many biology degrees these days
barely consider it) is that it illustrates the process of evolution with evidence of far more illustrative value than
the examples usually given in evolution courses. The truth is that this evidence is so extensive that it takes
years of study to begin to properly assimilate it. Too few scientists take the time and, as a result,
understanding of evolution is in decline, both among scientists and lay people. Molecular biology does also
furnish much evidence, but the detailed study of basic botany and zoology cannot be substituted. Another
reason for studying the invertebrates, apart from the fact that they are highly worthy of study for their own
intrinsic beauty and fascination, is the potential importance of these systems to engineering. It staggers me
that many learned people have no substantial interest in the inner workings of Nature's greatest creations -
living beings! Many people are fascinated by car engines or computers, as they should be, but Nature has
provided Earthlings with countless examples of extraordinary engineering, including engineering on the
microscopic and nanoscales. The invertebrates present to us many fascinating mechanical and computational
devices, some of which are being exploited in the manufacture of robots, but many more remain unknown to
the engineering community. You humans really need to open your eyes and look around you! The answers
that you seek are written in Nature's creations! Do not belittle a worm, it is an engineering and computational
feat far beyond the abilities and understanding of your best engineers!
OK, back to polychaetes! The first key to polychaete evolution is their segmentation and tagmatisation - these
have been discussed here. Nature constructs complex systems in the same way that engineers do - she takes
a design and uses it as a component in a more complex system. Take the wheel, its concept has been
replicated and put to many uses, for example a car not only has four wheels for traction, but it a steering wheel
and many rotating parts in its engine, gears and a drive-system. Complex computer programs, like operating
systems, are built from smaller modules, some of which are variously modified forms of an initial template
(would software engineers call class or object inheritance). Nature came up with a simple animal design and
then put these together into a chain, such that each animal module became a segment in a new more
complicated animal - a segmented worm. Now Nature had some spare segments to play with and joined some
together to form a head-end (cephalon, since many worms don't have definite heads!). The processors in
each of these segments was combined as a ganglion in a more powerful brain. Some of the chaetae became
modified as jaws and teeth. Light receptors were joined together into arrays, making more complex eyes, with
a brain nearby to process the data. Having lots of segments created new possibilities for hydrostatic skeletons,
assisting burrowing, crawling and swimming movements.
Nature didn't stop there! The ancestor of vertebrates, like human beings, as well as more complex animals like
insects and the octopus, had worm ancestors. If a nereis-like worm developed a more waterproof cuticle, then
it would be better able to venture out onto dry land (like a velvet worm) and if it developed joints in this cuticle
to move its parapodia, then it would have jointed legs and resemble a millipede. Likewise, a worm staying in
the sea could develop fin-like extensions (like arrow worms) and then it would be much like one of the ancient
jawless fishes (and it would already be red-blooded).
Looking in the other direction, the embryonic development of an animal can sometimes betray its past. It is
highly likely that polychaetes evolved from small ciliated creatures that swam around in the plankton, just as
the trochophore larva still does. Some ciliated worms are made from a few cells, and some single-celled
creatures are remarkably similar! Again, Nature came up with the cell, then joined cells together to make more
complicated organisms - complex patterns are often based on simpler repeating patterns.
Under construction! Links are still to be added below:
The current link is to an informative site that contains useful specific information on each polychaete species -
give it a browse!
Above: a 3D model of a pair of
segments of a typical errant polychaete,
as viewed from the anterior end.
Polychaetes have well-developed blood vascular systems. The diagram below illustrates the pattern of
vasculature in a typical segment in a typical errant polychaete showing detail in the right parapodium
(viewed from behind).
BC: blind-ending capillaries; DL: dorsal-lateral vessel; DV: dorsal (longitudinal) vessel; GLC: gut-lateral
connective (connects the gut plexus to the ventral-lateral vessel); PLC: parapodial-plexus-lateral connective
(connects the parapodial plexus to the ventral-lateral vessel); RB: recurrent branch of the ventral-lateral
vessel; VL: ventral-lateral vessel; VV: ventral (longitudinal) vessel. Also note the grid of vessels (gut plexus)
surrounding the gut and the parapodial plexus and the short connectives connecting the VV and DV to the gut
There is no typical heart or hearts in Nereis. Rather, all the larger blood vessels are contractile (by means of
smooth muscle cells that wrap around them). Blood flows toward the anterior head-end of the worm along the
dorsal vessel, DV (into the page in the above diagram) and toward the posterior tail-end in the ventral vessel,
VV (out of the page). Blood flows out from the VV into the pair of segmental ventral-lateral vessels (VL) which
supply each pair of parapodia. In the parapodia, these supply vessels branch repeatedly to form the complex
parapodial plexus which contains blind-ending contractile capillaries (BC). This parapodial plexus has a large
surface area and a large blood volume for gas-exchange, absorbing oxygen from the bathing sea-water and
losing carbon dioxide. The oxygenated blood is then drained into the DV via the dorsal-lateral vessel (DL).
Blood from both the VV and DV enters the gut plexus, where nutrients are absorbed from the gut. The gut
plexus carries blood toward the posterior end of the worm. Vessels branching from the DV also form a
body-wall plexus which supplies body-wall musculature and absorbs oxygen across the body-wall.
Note that there are two superimposed systems - in addition to the lateral segmental/parapodial supply, which
repeats in each segment, there is a longitudinal supply which carries blood along the length of the worm. The
segmental circulation tends to be much modified and reduced in the anterior-most ten or so segments, and in
the most posterior segments. The principal function of the DV, therefore, is to supply oxygenated blood to the
brain. In addition to the contractile blind-ending capillaries, there are more 'conventional' capillaries which
connect large vessels together, much as capillaries connect arterioles to venules in vertebrates. These latter
capillaries function in material exchange with the tissues and tend to be non-contractile.
Nereis virens is a slow swimmer. In the more active Nereis limbata, there are two separate blood-circulation
systems. One system, supplying the neuropodium, contains contractile blind-ending capillaries and is
presumably involved in gas exchange, whereas the remaining and separate part of the segmental circulation
has only the conventional exchange-type of capillaries. This increases the efficiency of oxygen uptake, an
adaptation for more efficient swimming, which parallels the double-circulation of mammals. In Nereis virens, the
contractile capillaries can interfere with circulation in other vessels, such that efficiency is not optimised 9even
though the contractile capillaries clearly enhance efficiency).
Superimposed on the description of circulation just given is a more erratic pattern, which illustrates areas
where the system is not as optimised as it is in, say, mammals. Blood-flow in a given vessel can sometimes
reverse direction, especially if blood-flow from two vessels opposes and flow may even be bidirectional within a
In addition to the blood-vascular system, polychaetes have an important coelomic circulation. Each segment is
bounded by an anterior intersegmental membrane and a posterior intersegmental membrane. Coelomic fluid
trapped within the coelomic compartment of each segment circulates by means of cilia that cover the coelomic
epithelial lining. Unusually, the coelomoducts do not open via pores to the outside and are instead modified to
form dorsal ciliated organs, paired on the dorsal roof of the coelom and which help circulate teh coelomic fluid.
A guide to polychaete species.